ph measurement and control device



Feb. 23, 1943. E. D. COLEMAN I PH MEASUREMENT AND CONTROL DEVICE Filed Jan. 25, 1939 6 Sheets-Sheet 2 Feb. 23, 1943. E. D. COLEMAN PH MEASUREMENT AND CONTROL DEVICE Filed Jan. 25, 1939 6 Sheets-Sheet 3 mwwwmww a I I I I 1 I 1 n I I Feb. 23, 1943. E. D. COLEMAN 2,311,976

PH MEASUREMENT AND CONTROL DEVICE Filed Jan. 25, 1939 6 Sheets-Sheet 4 25:9 are or Feb. 23, 1943. L N 2,311,976

PH MEASUREMENT.AND CONTROL DEVICE Filed Jan. 25, 1939 6 Sheets-Sheet 6 338 484' J5 J22 4a7 l T l .362

Patented Feb. 23, 1943 UNITED STATES PATENT OFFICE pH MEASUREMENT AND CONTROL DEVICE Edwin D. Coleman, Maywood, Ill. Application January 25, 1939, Serial No. 252,779

30 Claims.

This invention relates to ion activity measurement and control devices, such, for example, as a. hydrogen ion measurement device and parts thereof, and control means actuated thereby.

An object of the invention is the provision of a generally improved device for electrically measuring ion activity which is reliable both for scientific and control purposes and has greater ruggedness, accuracy, and simplicity in operation than similar devices heretofore known, and the combination therewith of improved recording and control means.

Another object of the invention is the provision of an ion activity measuring device having improved test electrode construction.

An object of the invention is also to provide an ion activity measuring device having improved cell chain construction.

A further object is the provision of an ion activity measuring device having improved means for bringing the test electrode and the solution into operative relationship.

Another object of the invention is the provision of an ion activity measuring device having electrodes capable of being easily and readily cleaned at the active surface thereof.

1 have also aimed to provide an ion activity measuring device having improved means for eliminating the effects of stray currents between the electrode supports which may interfere with the accuracy of the device.

A further object is the provision of an ion activity measuring device having improved means for supporting the test electrode.

Another aim of the invention is the provision of an electric ion activity measurement device having improved means for producing and for renewing the liquid junction thereoi'.

I have also aimed to provide an electric ion activity measuring device having improved means for storing the junction material and improved means for dispensing the same.

A still further object of the invention is the provision of an electric ion activity measuring device having improved relationship between the bridge material and the reference electrode.

A still further object is the provision of an ion activity measuring device having improved means for automatically and continuously or intermittently renewing the liquid junction.

Another aim of the invention is the provision of improved means for connecting the cell chain to an electronic amplifier.

A still further object is the provision of an ion activity test device embodying an electronic amplifier wherein means are provided for eliminating stray static interference and allowing operation of the instrument and its accessories 'without recourse to shielded leads or special shielded compartments.

Another object of the invention is the. provision of an ion activity test device wherein improved temperature compensation means is employed for automatically compensating for variations in the temperature of the test solution and electrodes of the cell chain.

A further object of the invention is the provision of an ion activity measurement device having an arrangement to accommodate improved bridge material.

A still further object of the invention is the provision of a bridge material of improved composition, such as to produce improved operating characteristics in a cell chain embodying the same and arranged to facilitate the renewal of the liquid junction either by manual or by automatic means during the operation of a testing device.

I have also aimed to provide a device of the class described having improved amplifying means.

A further object is the provision of improved means for feeding the output of the cell chain to the galvanometer or other measurement device.

A still further object is the provision of an automatic amplifying recorder having improved means for indicating the condition of the system.

Other objects and advantages will appear from the following description and the accompanying drawings, in which- Figure 1 is a wiring diagram showing one embodiment of my invention;

Fig. 2 is a horizontal section through the cabinets housing the electrodes and a part of the temperature compensation mechanism;

Fig. 3 is a vertical section through said cabinets;

Fig. 4 is a section on the line 44 of Fig. 3;

Fig. 5 is a section on the line 55 of Fig. 2;

Fig. 6 is a section on the line 6-6 of Fig. 4;

Fig. '7 is a section on the line 1-1 of Fig. 2';

Figs. 8 and 9 together show the invention as embodied in a pH indicating, recording and controlling device;

Fig. 10 is a section showing the mounting for the relay and switch controlling the input of test potential to the amplifier of Fig. 9;

Fig. 11 is an alternate form of galvanometer feed 0! F18- 9;

Fig. 12 is a view of a modified form showing means for continuously cleaning the test electrode;

Fig. 13 is a diagrammatic showing of means for automatically renewing the liquid junction, and

Fig. 14 shows a further way of connecting the reference electrode to the amplifier.

I have herein shown and described the invention as embodied in a device for the testing of solutions forpH, but it will be understood that many of the features thereof are not limited to the measurement of this particular property but may be applicable to the measurement and handling of other potentials of small magnitude and for analogous purposes. Two embodiments of the invention are shown, a manually operated pH measurement device shown in Figure 1, and an automatically operating device for indicating and recording the pH of solutions and for controlling the pH thereof shown in Figs. 8 and 9.

The cell chain and the temperature compensation elements are identical in both embodiments, and during the description of these elements reference may therefore be to either Figure 1 or Fig. 8, and the structures shown in Figs. 2 to 7, 12 and 14 are alike applicable to both embodiments of the invention.

Test electrode assembly Referring first to Figures 1 and 8, the cell chain includes a test electrode assembly designated generally by the numeral l2, a salt bridge l3, a reference electrode l4, and means for. maintaining the salt bridge and renewing the liquid junction including the element l5 and the stop cock IS. The structure of the glass electrode assembly is shown more in detail in Figs. 2 to 4, inclusive, and includes a glass electrode tube l1 and a temperature tube i8 (Figs. 1, 4 and 8). It will be understood by those skilled in the art that a metal electrode of suitable metal such as gold, platinum and antimony may be substituted for the glass electrode tube, in which instance the advantages of the mounting and sampling construction would still obtain, the chain then being sensitive to ion species determined by the nature of the sample and of the metal inner surface ofthe electrode, and where in the claims we use the term glass or test electrode, we also mean such equivalent where apv plicable. The glass electrode tube l1 and temperature tube I! are carried within a metal box designated generally by the numeral I! having a cover 2i secured across one face thereof by means of screws 22 which compress the cover against a rubber or similar seal 23 for the purpose of sealing the interior of the box against the passage of air therethrough for a purpose presently to be described. Secured in the bottom wall of the box l9 are fittings 24 and 25, these fittings .each having a hexagonal lower end and a cylindrical upper end 25 passing through openings in the box, the hexagonal lower end abutting against the outer surface of the box so as to prevent passage of air therethrough. The cylindrical upper end 25 is threaded for the reception of a nut 21 for fixing the fitting in place, a lock washer 25 being interposed between the nut and the inner wall of the box, the whole forming a practically airtight connection and providing electrical connection between the fittings and the box. The fittings have internal bores 29 and internally threaded counterbores 3| for the passage of liquid through the fitting and for the reception of inlet and outlet pipes 32 and 33. A further fitting designated generally by the numeral 34, which may be either of metal or of insulation material such as synthetic resin, is positioned against the top wall of the box l3 and has a longitudinal bore 35 extending from one end thereof to intersect a transverse bore 35. A second transverse bore 31 is provided adjacent the open end of the bore 35, and the open ends of the bores 35, 35 and 31 are normally closed by screw plugs 38, 39 and M threaded into the bores. The transverse bores 35 and 31 also have threaded counterbores 42 and 43 for the reception of nipples 44 and 45 which project through the top wall of the box l9 (note Fig. 4), insulation 45 and 41 being positioned against opposite sides of the box when the fitting is of metal for the purpose of electrically insulating the fitting and the nipples from the box. The nuts 48 and 49 are threaded onto the nipples and serve to secure the fitting rigidly in position on the box. The electrode tube I1 is supported within the box between the nipple 44 and the portion 25 of the fitting 24, the ends of the tube being in spaced relation to these elements and being secured thereon by flexible rubber sleeves 5i and 52. Metal bands 53 and 54 clamp the rubber sleeves to the fittings, and clamps and 55 likewise clamp the rubber sleeves to the ends of the electrode tube to provide fluid-tight connections of sufiicient flexibility so that the electrode tube cannot be strained or will not be broken by sudden shocks, either thermal or mechanical. The temperature tube i3 is likewise connected to the nipple 45 and the fitting 25 by means of rubber sleeves 51 and 58, and clamps 59 and 5|.

Intermediate the ends of the electrode tube I1 and preferably at the mid-point therebetween is a junction tube 52 joining with the electrode tube through which the salt bridge material is passed to establish the usual liquid Junction with the contents of the electrode tube, this lunction being formed at the point indicated by the numeral 63. Equally spaced from the point 53 are guard rings 54 and 55 on opposite sides of the point 53 and guard rings 55 and 51 equally spaced from the guard rings 54 and 55. Interposed between the guard rings 54 and 55 and the guard rings 55 and 51 and equally spaced therefrom are contact layers 58 and 59. I have found that intimate contact between the surface of the glass tube and the metal coating forming the guard rings and contact layers is imperative for consistently good results. This contact may be secured, for example, by thermally dispersing a metal such as silver over the glass surface preferably by spraying thereon. The characteristics of the metal are, however, exceedingly important if any material thickness is applied, since the film apparently puts the glass in a condition of stress. I have found that minimum asymmetry potentials are developed and superior stability occurs if lead is applied and is applied by the spray method. I have also used tin,

' brass, steel, zinc, and other metals, and have found that the performance appears to improve as the metal becomes softer. I have also employed silver and gold, applied chemically, both of which worked well but were difilcult to apply. Woods metal 'may also be employed where the temperatures of the solution are sufilciently low.

The solution under test is circulated through the interior of the electrode tube I1, as for examgarners ple, from a tank 54 by means of a pump ll. the solution passing through the fitting 2-4. the electrode tube It, the nipple 44, the fitting 24, the nipple 48, the temperature tube II, the fltting 2i, and thence back to the tank II or to some other point of disposal as circumstances may require. The fittings 24 and 2I-are in good electrical contact with the metal box I! so that these fittings are always at substantially the same potential and represent the only points at which the system is in electrical contact with the box, since the fitting 34 and the nipples 44 and 45 are carefully insulated from the box. Accordingly, if there is a diilerence in potential between the fluid lines delivering sample to fitting 24 or receiving sample from the fitting 25, this diiference in potential is largely dissipated by current flow between the fittings 24 and 25 through box l9, and there is a minimum of current flowing through the solution in the electrode tube, the fitting 34 or the temperature tube I 8. I have found this to be important as a means of eliminating serious errors. Furthermore, I have found that by insulating fitting 34 from the box I! any difference in potential between the solution within the fitting 24 and within the fitting 25 exerts much less error than would occur if fitting 34 were in electrical contact with the box I 9. It appears that no two fittings can be made having exactly the same potential with respect to the solution, and consequently there will always occur some now of current traversing the column of fluid within the electrode tube. When, however, the fitting 38 is insulated,'the two electrodes constituting the cell which generates this current will be fittings 24 and 25 and the electrolyte path becomes very long. Further, I have found it desirable though not essential that temperature tube it be made of somewhat smaller diameter than the electrode tube ll so that most of the IR drop will occur within the liquid in temperature tube it rather than within the liquid inside the electrode tube. It will be clear that the resistance of the fluid colunm within the temperature tube may be increased to any desired degree either by increasing the length or decreasing the bore of said temperature tube, and that the en'ect of the galvanic current in the electrode tube may thus be reduced to a satisfactory minimum. Further, it will be clear that the temperature tube not only may act as a support for the temperature compensating resistance coils but also may fulfill the even more important role of limiting said galvanic currents. I use the descriptive term temperature tube independent of whether the tube fulfills either or both of said-functions. The difference in potential between the fittings 24 and 25 and the solutions contacting the same can be largely eliminated if these fittings are made from pieces of metal immediately adjacent each other in the same bar, and can be further reduced by using approximately the same cutting action in boring the two pieces and in facing the inner surfaces of the same which are exposed to the fluid. Thus by using care and skill in the design and construction of the fittings 24 and 25, the error from this source can be almost completely eliminated.

In order to further reduce the effects of this transverse current through the electrode tube, I have found it desirable to make liquid junction with the liquid in electrode tube l1 at. the point 63 and to symmetrically space the contact layers 64 and I! with respect to the liquid junction at I! whereby the eitect of the transverse current on one of these contact layers is substantially compensated by the effect of the current on the other thereof. In other words, it the drop in potential between the liquid Junction 63 and the electrode layer 88 is positive, the drop between the liquid junction 63 and the electrode layer 69 will be negative and very nearly of the same magnitude. The result is almost perfect compensation so that when this construction is coupled with sym' metrical members 24 and 25 and with the use of a restricted bore in the elfect of the unavoidable galvanic currents generated between the fittings 24 and 2! and the contacting liquids becomes practically negligible so far as the operation of the device as a pH or anal. ogous measuring instrument is concerned.

The plugs 38,39 and 4| permit of access to the bore of the electrode tube and the temperature tube and to the bore 35 and allows insertion of a cleaning brush designated generally by the numeral 1| so that the surfaces of these bores may be readily freed from any adherent deposits or clogging which might occur therein. This is desirable particularly with respect to the electrode tube, since it has been found that certain suspended materials tend to deposit and adhere to the active surface of the electrode tube and thus cause the response of the electrode to become sluggish. When such a condition arises in an automatic pH controller, the period of response in the instrument may be so far delayed that automatic control becomes almost impossible. Similar deposits within the temperature tube It likewise retard the temperature response and also render the instrument inaccurate. The brush H may be periodically inserted in the bores of the tubes for manual cleaning thereof, but I have also found that the cleaning brush may be left permanently in place and the rod therefrom brought out through the bore I! through a suitable stufllng box (not shown). If desired, the cleaning brush may be driven with a reciprocating or rotary motion by means of a small electric motor so as to continously clean the active surface of the electrode tube when handling stubbornly adhering deposits, as shown in Fig. 12.

The salt bridge and reference electrode Liquid junction with the sample within electrode tube I! is established at the point 83 by means 01 a column of conducting material, preferably a semi-plastic material such as agar gel, presently to be described more in detail. In this instance, this column of conductive material includes the tube 62 connected to a tube 12 by means of a rubber sleeve 13 which is in turn connected to a tube 14 by means of a rubber sleeve 15, the tube 14 extending through a rubber grommet 16 positioned in the back wall of the box IE and in the side wall of a metal housing 11, the metal housing 11 being attached to the box ill by means of screws 18 and spacers 19. The tube 14 is connected to the stop cock l6 by means of a rubber sleeve M which is connected to the source of bridge material It and to the reference electrode l4 by means of tubes 82 and 83, this system of of tubes forming the conductive column or salt bridge I 3. The salt bridge tube I3 is substantially filled with the bridge material which constitutes an electrical conductor between the liquid Junction at 63 and the reference electrode i4. The stop cook it is normally closed, but a layer of bridge material is inevitably produced temperature tube II, the

about the 'bore of the stop cock in response to the turning of the cock, and maintains electrical communication through the cook. The free end of the tube 83 carries a fitting 84 supported in an insulating block 84a and having a bore 85 communicating with the interior of the tube 88, and a threaded counterbore 88 threaded to receive an adapter 81 which is also provided with a bore 88 adapted to receive the threaded dispensing end 89 of a conventional collapsible tube II. The bridge material is carried in the tube and'is dispensed through the bore of the adapter 81 and the bore of the fitting 84 into the tube 83. This arrangement is of considerable merit; because of the difficulty of attaching the collapsible tube 9|. I have found that because of the nature of the threads on the dispensing end 8| of tubesof this character, which are commonly made of soft metal such as lead, extreme difliculty is encountered in threading the tube into position. I have therefore provided the adapter 81 which a fluid. It will be seen that I have not only provided a novel form of reference electrode, but In addition, novel means for re-making the liquid junction, and this, in combination with the use of a sealed reference electrode, constitutes a highly improved and simple procedure for replacing the is screwed onto the end of the tube prior to at- 'tachment into the fitting 84 and the adapter is subsequently screwed into the counterbore 88, thus permitting the adapter to be applied to the tube in the open where greater care and accuracy can be obtained in the fitting of'the parts.

The bridge material is exposed to the sample in the electrode tube I1 at the point 83, and as this surface continues to be so exposed, the concentration of KCl or other salt contained in the bridge material is gradually reduced until eventually appreciable error would be developed at 63 unless means were provided for replacing this surface with a fresh surface of bridge material sufilciently rich in KCl. This is accomplished by opening the cock I8 and then compressing the tube 9|, thereby forcing bridge material from the tube into the salt bridge tube I3, discharging an equivalent amount of the exhausted bridge material at the point 63 from whence the exhausted material is rejected from the electrode tube with the flow of sample therethrough. The cock I8 is closed during normal operation so that in the event of excessive fluid pressure in the electrode tube, this excessive pressure cannot be relieved by driving the bridge material back into the collapsible tube 9| or into the reference electrode I4.

The reference electrode is formed directly on the end of the tube 82 (see Fig. 5) and consists of an elongated glass bulb 92 having sealed in the bottom thereof a platinum or other suitable electrode 03 immersed in mercury 9| covered in turn with a layer 05 comprising a mixture of calomel and KC]. Positioned thereabove is a solution 98 consisting of saturated KCl solution which fills the bulb and extends through the bore of the tube 82 to approximately the point 91 where contact is made with the bridge material. At the point where the bulb joins the tube 82 a small piece of glass wool 98 is fused into the wall of the electrode bulb to prevent the mercury from flowing into the tube 82,'thus escaping from the bulb. The glass wool further functions to prevent granules of the bridge material from passing through the tube 82 and falling into the electrode proper. This assembly constitutes a novel type of saturated calomel half cell, but it will be understood that my invention is not limited to any particular form of half cell, but also contemplates in certain phases the use of half cells of other types, such as the unsaturated calomel half cell or other half cells well known in the art, or similar devices for producing substantially constant potential between a metallic conductor and i liquid junction without danger of contaminating the reference electrode and with a minimum of difliculty, time and effort. I have also provided novel means for storing a reserve supply'of bridge material, and novel means for replacing the pp y.

Compensation for change of pH of test solution with change in temperature thereof It is known that practically all solutions undergo changes in pH with change in temperature, and therefore, unless some correction is made the pH indicating instrument will be responsive both to the pH of the sample and the temperature thereof, even though perfect thermodynamic correction is applied to the remainder of the electrical system under the well recognized equation According to my invention, means are provided which can be adjusted to fit practically any type of sample solution and to bring about correction for this temperature effect over reasonable pH ranges sothat the ultimate reading of the pH instrument will be substantially independent of the temperature of the sample and will indicate the pH that this sampl would have were it at the normal pre-selected temperature instead of indicating the actual pH of the sample at the temperature of the test.

In order to effect compensation for variation in temperature of the sample and for variation in temperature of the reference electrode, two temperature compensating circuits are employed, designated generally in Figs. 1 and 8 by the letters A and B. The reference electrode is surrounded with a resistance coil 99 (Fig. 5) having leads MI and I02, and over the entire-assembly is mounted a protective sheath I03, the resistance coil 88 being temperature sensitive. A lead I00 connects to the platinum terminal 93 of the reference electrode and constitutes one terminal of the cell chain consisting of the reference electrode and the glass electrode, as will presently more fully appear. The lead I04 from the reference electrode is connected at the point I05 to a network comprising resistances I06, I01, I08 and I00 and the-temperature sensitive coil 99, the former being mounted on a plate I II within the housing 11 and constituting a Wheatstone bridge fed by a battery H2 likewise secured to the plate III by means of a bracket I I3 having a thumb screw I I4 for easy replacement thereof. A resistance I I8 is interposed between one side of the battery and the bridge. Conductors H8 and H1 connect intermediate points H8 and I I9 of the bridge to a double-pole; double-throw switch designated the bridge, a difference in potential substantially equivalent to the increment in potential occur-- ring in the cell chain by virtu'e'of the temperature change of the reference electrode. In this particular instance the condition is met when resistances I06, I01 and I09 are wound from manganin wire and are 500 ohms each while resistance I is 400 ohms of the same wire and temperature responsive resistance 99 consists of a nickel coil having a temperature coefficient of substantially 0.005 and a resistance of substantially 100 ohms at 25 C.

The Wheatstone bridge circuit A is therefore in series with the reference electrode. Connected to the lead I02 and thence to the point I23 of this bridge circuit is a conductor I24 connected at point I25 to the second Wheatstone bridge circuit B comprising resistances I26, I21, I28 and I29, the resistance I29 being wound on the temperature tube I8 and being in this particular instance of nickel wire of approximately 0.005 temperature coefiicient and having a resistance of 500 ohms at approximately the temperature at which the sample will normally be. The other resistances are temperature insensitive coils, in this instance of 500 ohms each. Means are provided as heretofore described for the ready removal of the temperature tube I8 so that a suitable value for the resistance I29 may be selected for the particular temperature at which the instrument is intended to operate and so that the resistances may be readily interchangeable. Conductors I3I and I32 connect the temperature responsive coil I 29 to the remainder of the bridge and include a connector member for conducting the current from the housing 11 to the box I9. This includes rubber grommets I33, a spacer I34 of insulation material within which is provided a tube of insulation material I35 carrying a smaller tube I36 of metal contacting at its ends with plates I31 on the ends of the conductor I32. Within the metal sleeve I36 is an insulating tube I38 carrying a threaded rod I39 having nuts I4I for attachment of the ends of conductor I3I, the conductors being separated by spacers I42 and I43.

The latter Wheatstone bridge circuit is fed at the points I45 and I46 through conductors I41 and I48 connected to a polarity reversing switch designated generally by the numeral I49, this switch being fed through conductors II andI52 from a battery I53 and a voltage divider I54 having a movable slider I55, actuated'by a knob I56 carried on the panel III. It will be seen that the position of the slider I 55 will fix the potential applied to the bridge so that the degree of response of this network with respect to the temperature at the coil I29 is adjustable both as to magnitude and as to polarity. Conductors I51 and I58 connect the polarity switch I49 to the switch I2I in such manner as to measure on the volt meter I22, the potential being applied to the second bridge circuit, that is, the circuit designated 3. whereby the reading of this meter becomes a measure of the temperature response of said bridge. The purpose of this bridge is to compensate for.changes in the pH of the sample with change in temperature. By properly adjusting slider I55 it is also possible to adjust the circuit so it compensates not only for change in pH of the sample with temperature, but will also compensate for any theoretical change in a normal hydrogen electrode at solution temperature.

Referring now more particularly to Figs. 2 and 3, the batteries I I2 and I53 are shown mounted on the panel board I I I. The voltage divider knob i indicated at I56, and the polarity reversing switch at I49;the switch I2I is shown as a push button normally connected to conductors I51 and I58, but connecting the meter I22 to the conductors H6 and H1 when depressed, thereby permitting momentary checking of the potential delivered from the battery II2. While the potential of battery II2 may vary somewhat, it is permissible from a practical standpoint to allow some variation in the potential thereof without introducing serious errors in the measuring circuit, since the temperature correction to the reference electrode is small in any event, and by means of the button I2I the potential of the battery may be readily measured at any time and the battery replaced when necessary. In the released position, with the meter I22 connected to the conductors I51 and I58, the meter continuously indicates the degree of correction being applied by bridge B, and by means of the voltage divider I54 this correction can be periodically re-set as the battery I53 gradually fails.

Attention is now directed more particularly to the manner in which the above-described elements are assembled, shown in detail in Figs. 2 and 3. It will be noted that the electrode tube I1 and the temperature tube I8, together with their directly associated parts, are assembled within the box I9. It will be realized that the sample being circulated through the electrode tube and temperature tube may be at temperatures substantially different from room temperature, and since the temperature of the sample may vary to a considerable extent, I have found it desirable to thermally isolate the reference electrode from the region in which the temperature of the sample may exert any material effect. This is accomplished by mounting the electrode tube and the temperature tube within the box I9 and by providing the cover 2I and the gasket 23 to substantially hermetically seal the box. Furthermore, space is provided within the box for the reception of an open container of a desiccant such as fused calcium chloride, hydrous calcium sulphide, sulphuric acid, or other desiccant to substantially free the air within the chamber from moisture. This is of particular importance where cold solutions are being passed through the electrode tube since otherwise condensation of moisture occurs on the outside of the glass electrode that may seriously interfere with the operation of the instrument because of short circuits between the various parts thereof. Furthermore, the arrangement is such that all adjustments and the cleaning of the electrodes is possible without openings thereabout and prevent infiltration of moisture. The housing 11 is provided with a door, as indicated at I 59 (Fig. 2) provided with a. gasket I 6| and secured in place by a wing nut I62 which may be opened to allow immediate access to the interior thereof for making adjustments in the electrical circuit as heretofore indicated. I

In order to conduct the current from the glass electrode tube to the measuring instrument, wire loops I63 and I64 extend around and in intimate contact with the contact layers 68 and 69 and are engaged by a conductor I65 (Fig. 3). loops I66, I61, I68 and I69 shown in Fig. 4 likewise engage the guard rings 64, 65, 66 and 61 and are interconnected by a conductor "I (Fig. 4). The conductor I65 is connected to a metal rod I12 which extends from the interior of the Wire box I3 through the housing 11 as shown in Fig.

the measuring instrument through a cable I13,

the lead I13 having a shield I15, so designed that the potential of the shield I is at the same potential as the lead I13 when the potentiometer of the measuring circuit balances that of the cell. The shield I15 is connected to a conducting plate I13 through a terminal I11. Mounted to the plate I13 and in electrical contact therewith is a metallic tube I13 protected from contact with outside sources by an insulating sleeve I13 and insulated from the rod I12 by a glass tube I3I. The metallic tube I13 makes contact with a metallic washer I32 within the box I3 to which is connected the conductor Hi to which the guard rings are in turn electrically connected, these elements forming part of the so-called McClure circuit, which will presently be more fully described. Ceramic insulators I33 and I33 are in contact only with the conductor "I or with regions at the same potential as the conductor I13 and the shield I15, and therefore, no leakage can occur from the ceramic insulators due to surface leakage when the cell changes potential balance because ,there exists no diflerence of potential between the conductor I1I or the lead I13 and the adjacent portions of the McClure circuit including the elements I82 and I13. It is thus possible to connect the glass electrode to the measuring device through the shield I15 without in any way disturbing the potential of the glass electrode lead including the conductor I35, the rod I12 and the lead I13 by stray electrical leakage, and it is further possible to extend the cable I13 for distances of several hundred feet, since a cable of this construction cannot leak to the conductor I13 when used in the electrical circuits herein described.

The purpose of the guard rings 33 to 31, inclusive, is to prevent electrical leakage along the surface of the glass tube. These four guard rings are connected together and to the McClure circuit, and hence, since the potential of this circuit is the same as that of the contact layers 33-and 33 of the glass electrode at the point of balance, there can occur no leakage from the contact layers along the surface of the glass tube.

Potentiometer assembly and thermodynamic temperature compensation of Figure 1 The temperature compensation heretofore described functions to correct for the actual change in pH of the sample which occurs with Connection is made to the reference electrode through a conductor I35 (Fig. 1) connected to the circuit of the Wheatstone bridge B at I33, the preferred manner of making this connection being shown in Fig. '1, the conductor being preferably enclosed within the cable I13 and being connected to a rod I31 in a ceramic insulator I33 carried on and insulated from the plate I13. A conductor I33 within the housing 11 connects to a binding post I3I on the control board III (Fig. 3) and thence to the point I33 of the bridge circuit. A cover I32 on the side of the housing 11 covers the insulators I33, I33 and the respective connections. The potential generated by the cell chain and impressed n the conductors I13 and I35 is measured by'means of a potentiometer assembly shown in Fig. 1, in

which the potentiometer circuit is designated by a resistance I33 connected by a conductor I33 to a potentiometer resistance I33 having a slider I33, to which the conductor I35 from the reference electrode is electrically connected, a variable resistance I31 connected to the resistance I33 by means of conductors I33 and I33, 9. resistance 23I connected to the potentiometer resistance I 35 by conductors 232 and 233 and to variable resistance I31 by a conductor I33, a

resistance 233 connected to the conductor I33 by means of a conductor 233, a variable resistance' 233 having a slider 231 connected to the conductor 233 by means of conductor 233, a battery 233, one terminal of which is connected to the resistance 233 by means of a conductor,

2I I, a switch 2I2, one side of which is connected to the other terminal of the battery 233 by aconductor 2I3, the opposite side of which is connected to the resistance 233 by a conductor 2, a switch 2I5 having two positions indicated by the letters Std. meaning Standardized, and the letters Op, meaning Operate, the Std. position connecting a McClure circuit indicated by the numeral 2I3 either to the conductor 2 by way of conductor 2I1 or to conductor 2I3 attached to the slider 2I3 of variable resistance I31. A standard cell 22I is connected to conductor I33 and has a lead 222 connected to a switch 223 through a contact marked Std. and having the mentioned significance, the switch serving to connect a conductor 223 either to the conductor 222 from the potentiometer assembly or the conductor I13 leading from the glass electrode. The conductor 223 is connected to the amplifier or other voltage detecting device presently to be more fully described, but which functions for the purpose of determining when the potentiometer assembly is brought into electrical balance with the cell chain. The standard cell 22I is connected to Op se the potential of resistance 233, and in balancing the bridge, the two potentials are exactly balanced by moving slider 231 of resistance 233, the point of balance being indicated by the amplifier assembly presently to be described. In parallel with the series connected resistance I33 and potentiometer I35, 8. further resistance 223 is connected consisting of a temperature sensitive coil connected to conductors 235 and 233 of the potentiometer circuit, the coil 225 being positioned on the temperature tube I3 and being thermally responsive thereto. As the resistance of coil 225 changes with temperature, the potential drop between conductors 235 and 233 will alter, providing the potential drop through resistance 233 is constant. I have found, by way of specific example, that if the combined resistance between the conductors 235 and 233 of the network including the resistance I33, potentiometer I35, variableresistance I31, and resistance temperature coefllclent of 0.005, then if the resistance of coil 225 is 46.8 ohms at 25 C., the potential drop between the conductors 205 and 203 will follow closely the absolute temperature of the temperature tube I9, that is, it will follow the equation Furthermore, if the resistance 206 and the potential of the battery 209 are made large, the effect of change in resistance at the coil 225 on the change in current through the resistance 204 becomes so small that negligible error oc- .curs even when no change in adjustment is made at the variable resistance 206 over wide changes in temperature at the temperature tube. Thus, this potentiometer assembly becomes responsive to the temperature of the sample according to thermodynamic requirements.

When the potentiometer circuit is balanced or standardized, the switches 2I5 and 223 are moved to the operate position and the potential of the cell chain is then opposed to the potential between the point of the slider I96 and the point of the slider 2I9. By moving slider I96 a point can be reached on the potentiometer at which these two potentials exactly balance out as indicated by the amplifier assembly, and the potentiometer I95 being calibrated in therms of pH, the pH of the solution can be read directly therefrom. It will be noted that as the temperature of the sample varies and the potential of the cell chain changes in accordance therewith, the potential between the slider 2I9 and the slider I96 will alter in the same manner so that the instrument reading will be substantially constant, irrespective of temperature changes in the sample, provided there does not occur a concurrent change in the pH of the sample.

Where the cell chain consists of a saturated calomel half cell coupled with a glass electrode consisting of Coming 015 glass having a contact layer of lead, its potential will be reasonably close to zero at zero pH, under which conditions the slider I96 would be at the extreme end of the potentiometer resistance adjacent to the resistance I93. The value of resistance I91 is then made to be approximately twice that of resist- 'ance I93 so that it is possible to apply either a positive or negative asymmetry correction. The purpose of potentiometer I91 and slider 219 is to apply to the potentiometer circuit a fixed increment sufllcient to compensate for the asymmetry potential of the glass electrode, while the purpose of resistance I93 is to balance the potential range of the potentiometer I 95 to coincide with the potential range of the glass electrode cell chain.

Amplifier of Fig. 1

The impulse amplifier shown in Fig. 1 functions to determine the condition when the potential of the potentiometer assembly is equal and opposite to that of the cell chain and for the purpose of standardizing the bridge, depending upon the position of the switches 2| 5 and 223, the procedure so far as the amplifier is concerned being identical. Assuming the switch to be in the Operate position, the glass electrode lead I13 is then connected to the conductor 224 to which connection is made through a resistance 226 to a switch 221 having a contact 228. Contact 228 is accordingly in series with the cell chain by way of conductor I13 through the glass electrode, through the liquid Junction and the salt bridge l3 to the reference electrode I4. thence through the temperature correction bridges A and B, thence through conductor I to the potentiometer I95, from the potentiometer I to the slider 2I'9 of resistance I91 and from slider 2I9 through switch 2I5 to the McClure circuit. The opposite contact 229 of the switch 221 is likewise connected to the McClure circuit by conductor 23I. The switch 221 has a movable switch blade 232 connecting through a condenser 233 to the grid 234 of a vacuum tube 235. The filament 236 01 this tube is also connected to the McClure circuit through a resistance 231, a switch 238 and a conductor 239.

It will be apparent that if the potential between the sliders 2I9 and I96 is not equivalent to the output of the cell chain as modified by the temperature compensators, there will be a diflerence in potential between contacts 226 and 229. Switch blade 232, normally engages contact 229 and when moved from this position to engagement with contact 228, if such difi'erence in potential exists, there will be a change in potential on element 24I of the condenser 233, and this will be transferred and expressed as a pulse or change in potential on the grid 234 of vacuum tube 235. On the other hand, when the potential between the sliders I96 and 2| 9 of the potentiometer assembly is equal and opposite to that of the cell chain as modified by its temperature compensators, contacts 228 and 229 will be at the same potential, and under these circumstances switch Made 232 may be moved from engagement from contact 229 to 228 withoutaifectlng the potential of the grid 234. The vacuum tube 235 is resistance connected as the first step in a cascade including additional tubes 242 and 243 arranged as will be apparent from the drawings, and in the plate circuit of the third tube 243 of this cascade a meter 244 is connected. It will be seen that as the potential of the grid 234 of the first tube is altered by movement of the switch blade 232, the change will be reflected on the meter 244, and it will further be seen that with any given pH of the sample passing through the glass electrode I1, there will be a position of the potentiometer slider I96 at which the potential generated by the sample and modified by the temperature correction bridges will be exactly balanced, under which circumstances there will be no change in the reading of the meter 244 as the switch blade 232 is moved from contact to contact, and by this means, therefore, the pH of the solution can be read from the potentiometer I95.

An alternative construction is shown in Fig. 14, wherein the conductor I13 is continuously connected to condenser 233 and thence to grid 234 of the input tube. A switch 232a is normally open as indicated, under which condition the plate of condenser 233 is at the same potential as con- When switch 232a is closed, the potential on said condenser will alter to that of conductor 248, and there will occur a change in potential in grid 234 unless the potential and all potentials exactly balance, in which case conductors I13 and 248 will be at the same potential and no change in potential of grid 234 will occur as switch 232a is functioned. Condenser 214a shunts switch 232a and serves the dual function of absorbing any contact potential generated at contacts when switch 232a is closed and decreasing the rate of change of potential of connector plate of condenser 233 when switch 232a is opened, thus eliminating extraneouspotentials in both operations.

Inpractice, it is frequently desirable to locate the potentiometer and amplifier at some distance from the cell chain proper, and under these circumstances potentials may be generated in conductor I13 by virtue of electronic or electro-magnetic fields, and if said potentials are transferred to grid 234 when switch blade 232 is moved to the contact 228, there will occur in meter 244 reactions independent of their relationships to the potential of the cell chain to that of potentiometer, that is, the instrument will become sensitive to stray fields. I have found, however, that the effect of stray field interference can be almost completely nullified by utilization of a high i-mpedance located at 225 and condensers 245 and 246. Such a combination constitutes a filter network, and I have found that when resistance 226 is approximately 10 megohms, and condensers 245 and 246 are approximately 0.002 microfarad, that even extreme interference can be eliminated.

It is to be noted that both condensers 245 and 246 return to a point at McClure potential, and it'will be recalled that McClure potential is the same'as the potential of conductor I13 at the condition of balance. Consequently, under these circumstances. it is impossible for electrical leakage to occur through condensers 245 and 246. I have also found it advisable to use a third condenser 241, one side being connected between the switch blade 232 and the condenser 233, and the other side connected to the McClure circuit as indicated at 248. I have found that by so doing extraneous contact potentials produced when the switch blade moves between the contacts 229 and 228 are largely eliminated. The condensers 245, 246 and 241 are in this instance 0.002 mf. condensers.

pH recorder and controller In Figs. 8 and 9 I have shown the invention as embodied in an automatic indicating, recording and controlling mechanism wherein the cell chain and the temperature compensation bridges A and B are identical with the form shown in Figs. 1 to '7, inclusive, and wherein the potentiometer assembly and thermodynamic temperature compensation is substantially similar in principle to that of Figure 1 with the exception of certain changes in the circuit necessitated by the additional functions to be performed. This embodiment includes a potentiometer assembly designatedgenerally by the numeral 248, a power operated switching recording and potentiometer adjusting mechanism indicated generally by the numeral 249, an amplifier system I (Fig. 9) and a power supply 252.

Potentiometer assembly Fig. 8

This potentiometer assembly is similar in principle to that of Fig. 1 and employs the resistance I93, the slide wire potentiometer I95, the resistances 204 and 20I and the variable resistances I91 and 206 arranged in the same relationship. However, in this instance the slider 2I9 of the resistance I91 is connected to McClure circuit by means'of a conductor 253, in this instance connected to the metallic shield I15 around the lead from the glass electrode. Furthermore, the standard cell 22 I is in this instance connected into the conductor 2 between the resistance 204 and the working battery 209, and is connected to a switch 254 having two positions designated as standardized and operate, the common pole of this switch being connected to a galvanometer as V asnmc having an open position for operation and a closed position for standardizing is interposed between the conductor I99 of the potentiometer assembly and a conductor 256 of the galvanometer circuit. The potentiometer circuit in this instance also has a temperature responsive coil 225 heretofore described and functioning in the same manner. In this instance the slider I96 of the potentiometer I is mechanically operated by suitable means from the driving mechanism 249, in this instance shown as a rod 251. A galvanometer 258 is, in this instance, used to balance the potentiometer circuit and is connected across resistance 204 by placing switches 254 and 255 in the standardized position in which they connectto the galvanometer through conductors 259 and 26I. In the Operate position the galvanometer is fed by wayof conductors 255, 25I, 259

and 282 through elements presently to be described from the output of the impulse amplifier 25I. Resistors 263 and 264 are used to adjust the galvanometer needle in accordance with the response of the particular amplifier and galvanometer coil employed, as will be understood in the art, the resistor 263 being known as a critical dampening resistance.

Motor operated switching unit The motor operated switching recording and potentiometer adjusting unit 249 includes a motor 255 arranged to drive a shaft 266. The shaft 266 drives a further shaft 268 which in turn has a gearing 289 driving a shaft 21I, which in turn drives a sprocketed roller 212 through gearing 213. The roller 212 has a sheet of recording paper 214 trained thereover from a supply roll 215 upon which the results of the test are continuously recorded by a pen 216 or other analogous device well known in the art. A cable or cord 211 is trained over pulleys 218. 219, 28I and 282 and carries the pen 216, the cable being given a turn around a pulley 283 positioned on the shaft 251 so that the pen will be moved longitudinally of the roller 212 in one direction or another in response to rotation of the shaft 251 so as to produce a line on the paper 214 in response to continuous movement thereof under the action of the motor 265. The mechanism just described, together with certain parts of the following description, is a recording mechanism of well known design such as that shown in Fig. 6 of Keeler Patent 1,472,125, issued October 30, 1923. It will be understood that this is only by way of illustration, and the principle of the invention may be applied to any well known type of recording device, numerous of which are known to the art. In this particular form, the shaft 268 carries spaced cams 284 and 285 fixed thereon and intermediate cams 286 and 281. A lever 288 is pivoted near its upper end and depends in a position to be actuated by the cam 281, and an arm 289 is pivoted thereon on a horizontal axis adjacent the lower end thereof, each end of the arm carrying a shoe 29I of cork or other suitable material for frictionally engaging the rim 292 of a clutch disk or wheel 293 fixedly secured on the end of the shaft 251. The cam 281 periodically engages the lever 288 to move it outwardly away from the disk 293 'in opposition to a spring, not shown, thereby lifting the shoes 29I free from the rim 292 to permit rotation of the arm under the influence of the cam 281, the spring returning the shoes into engagement with the rim upon release from the cam. After the cam 281 has lifted the shoes from the rim, the cam 286 engages the end 294 of the lever 295 which is fixedly secured at its upper end to a bar 296 pivoted at its opposite ends as shown at 291. A jaw member 298 is secured to the upper edge of the bar 296 and has an upper edge which is inclined and increases in height toward each side as shown at 299. The needle 39l of the galvanometer 259 extends immediately above the edge 299, the needle being deflected by the movable coil 392 or other operating device of the galvanometer or analogous potential measuring mechanism. Abutments such as that shown at 393 at the ends of the surface 299 serve to limit the deflection of the needle 39L Above the needle, and beneath which it normally freely swings are the jaws 394 and 395 preferably having straight and horizontal lower edges, the jaws being pivoted as shown at 396 and extending toward each other leaving a gap of suflicient width between their inner ends to allow free entry of the needle 39| when in mid position, the needle normally swinging between the edges 299 and the lower edges of the jaws 394 and 395, the latter of which have downwardly extending arms 391 and 398 drawn toward each other by a spring 399. Attached to the lower end of the lever 288 is a plate 3| l carrying pins 3|2 and 3|3 cooperating with the lower ends of members 391 and 398. At opposite ends of the arm 289 are lugs 3l3 and 3 adapted to be engaged by the cams 284 and 285. In operation, the needle 39| may be deflected either toward the right or left facing Fig. 8 from the central position. Assuming, for example, the needle to be deflected toward the right, the periodical vertical movement of the bar 296 by cam 286 driven by shaft 268 permits the needle to move between the edge 299 and the lower edge of jaw 394, in which position it will be clamped causing the right hand arm 391'to be tilted on its pivot in a clockwise direction, thereby pushing on the pin 3l2 of plate 3 to tilt the movable or driving clutch arm 289 in a clockwise direction while cam 281 holds the shoes 29| away from the rim 292, the extent and direction of angular movement of the member 289 being dependent upon the degree of deflection of the needle 39| under the influence of the galvanometer, and consequently dependent upon the extent of unbalancing of the circuits, as will presently appear.

The cam 281 shortly releases the shoes 2!" in response to further rotation of the shaft 268, which again grip the ring 292. With continued rotation of the shaft 268 the cam 285 engages the lug 3l4 pushing it downwardly and restoring am 289 to horizontal position during which movement the arm 289 rotates the disk 293 in counterclockwise direction, thereby rotating shaft 251 in like direction. During this movement the slider I96 of potentiometer I95 is likewise moved through an equal arc so as to re-balance or tend to re-balance the potentiometer circuit. During this movement of the shaft 251, the pen 216 is likewise given corresponding movement on the chart 214. When the needle 39! moves to the left from this central position, analogous operation occurs, causing rotation of the shaft 251 in the opposite direction. During this movement of the shaft 251 under the influence of the galvanometer, a brush 315 is brought into engagement with one or other of segments 3; and 3" connected by conductors 3! and 3l9 to a motorized valve 32!, the brush 3| 5 likewise being connected to the motorized valve through conductors 322 and 323 and battery or other source of power 324. The motorized valve 32l is of the type common in the art having a valve proper and a motor arranged to index the valve in either direction to increase or decrease the flow therethrough. The valve 32l controls the flow through a pipe 325 from a source of supply 326 of a reagent for ad- Justing the pH of the solution in the container 1|. It will be understood that the valve 32l and associated circuitare by way of illustration, and that there may be substituted therefor the usual and common types of control mechanism, alarms or signal mechanism to be operated in response to change in pH of the sample.

Mounted on the end of the shaft 268 is a cam 321 positioned thereon in such manner as to depress a multiple switch 328 during the interval when cam 281 has the shoes 29l raised from the rim 292. The switch 328 consists of blades supporting contacts 329, 331 and 332. In the position shown in Fig. 8, galvanometer 258 is open except for its critical dampening resistance 263, while feed condensers 333 and 334 (Fig. 9) are shorted through contacts 33l, conductors 335 and 336 and resist anee 331, which resistance is substantially equal to the resistance of the galvanometer and its critical dampening resistance 263. As switch 328 is actuated by the cam 321, contacts 331 are opened, and contacts 332 are closed, thus connecting the galvanometer and the critical dampening resistance with condensers 333 'and 334 (Fig. 9) which are included in the output circuit of the amplifier. Subsequent to closure of contacts 332 there occurs closure of contacts 329,-

which produces a pulse in the amplifier circuit if the potential circuit is not in balance by means hereinafter described. At the same time resistance 331 is taken out of the galvanometer circuit. This method of switching has a decided advantage in that a load is applied to the condensers 333 and 334 at all times so that there is no opportunity foran electrical charge to build up when these elements are not connected to the galvanometer, while at the same time the galvanometer is allowed to return to its mechanical zero. By substituting the resistance 331 for the galvanometer during the remainder of the cycle the galvanometer is allowed maximum time in which to return to rest, while any potential applied to the condensers subsequent to the first pulse delivered from the amplifier is dissipated in the resistance 331, and this circuit comes to equilibrium before the galvanometer is again connected on the next cycle. Accordingly, at the instant contacts 332 are closed, the galvanometer is in a condition of mechanical rest and there exists practically zero potential between conductors 335 and 336. The amplifier has had opportunity to return to normal subsequent to the previous cycle, and the entire system is prepared to respond to any difference in potential that may exist between the cell chain and the potentiometer circuit at the instant they are connected to the amplifier system.

Amplifier and power supply system of Fig. 9

Connection of the cell chain and the potentiometer circuit to the amplifier system occurs when contacts 329 are closed, the blades being set so that this occurs immediately subsequent to closure of contacts 332. Closing of contacts 329 completes the circuit through a relay designated generally by the numeral 338 shown more in detail in Fig. 9 from true ground by way of conductors 339, 34!, contacts 329, a conductor 342, a resistor 343, relay 338, a conductor 344 to one side of a source of direct current, indicated in Fig. 9, by rectifier 345, and thence through conductors m, an and m to true ground. Relay-336 is also continuously energized from rectifier 345 by way of conductors 349, I, 352 and 353, resistance 354, relay 336, and conductor 344, the energy thus continuously fed to the relay being insufficient either to close or to maintain the relay in the closed position unless current is also fed to the relay through contact 329, as above described. This construction minimizes the fiux increment in the relay necessary for operation thereof, thereby minimizing extraneous potentials from this source in the grid circuit of the by mounting on an insulating block 346 which in turn is mounted to the case 346a connected to true ground. Inasmuch as the frame of the relay and the assembly supporting the relay switch blades are at McClure potential at the point of potentiometer balance, there can be no leakage from these switch blades for reasons already explained. Further, I have found that a relay of the so-called telephone type as shown in Fig. 10 is particularly suitable for this purpose since its metal frame isparallel to the switchblades and magnetic flux leakage from said frame will, therefore, induce the minimum potential in the switch blades when said flux is altered.

The grid 355 of the first tube designated generally by 356 is connected to ground through conductors 351 and 356 and grid leak 359 and is connected to one contact of the relay 336 through conductor 351, condenser 36I, and switch element 362. In the normal de-energized position of the relay, the switch element. 362 contacts switch element 363 connecting the grid-355 to the McClure circuit by way of conductor 351, condenser 36I, contacts 353, conductors 364, 355 and 366. A condenser 356 connects the McClure circuit to true ground by way of conductors 366 and 365, a conductor 356a, condenser 356, a conductor 35617, a conductor M1 and conductors 352, 35I, 349, 346, 341 and 346. The purpose of this condenser is threefold: first, it completes the cell circuit to the ground side of grid leak 359 by way of conductor 6; secondly, it by-passes any static charges or transients that may develop in the circuits external to the amplifier 25I, and thirdly, it acts as a blocking condenser, protecting the reference electrode I4 (Fig. 8) from becoming short circuited in the event that the solution in the tank 56 becomes grounded. when contacts 329 close, relay 336 is energized and completes circuit through the switch 362 by way of contact 361 connecting condenser 36I to glass electrode lead I13 by way of conductor 366 and resistance 369. When the potential of the po- The relay is operated under conditentiometer assembly balances the potential of the cell chain as modified by the temperature compensation bridges A and B, the potential of the conductor I13 and of the shield I15 is. equal, and under these circumstances when switch 362 moves from engagement with contact 363 into' engagement with contact 361, no change in potential is imparted to the condenser 36I, and

there is accordingly no change in potential ererted on grid 355 of tube 356. However, if the output of the potentiometer assembly fails to balance the potential of the cell chain as modified, then when switch 362 is actuated to move out of engagement with contact 363 and into engagement with contact 361, a charge will be imparted to the grid 355 from condenser 36I, and this charge will be amplified through the cascade including the vacuum tubes 356, 3" and 312, and will appear as a difference in potential at condensers 333 and 334, causing a diiference in'potential between conductors 335 and 336-which difference is transferred to the galvanometer 256 a by way of conductors 335, 256, 25I, coil 362, conductor 259, switch 254, conductor 262. resistance 264, contacts 332 and conductor 336. The resultant swing in the galvanometer needle "I occurs Just prior to the lifting of the chopper bar 296. Thus, as the cam 321 actuates switch 326 there will, at unbalanced condition of the instrument, simultaneously occur a pulse of energy through the galvanometer resulting from the out of balance potential amplified through the eascade, and this-pulse will anove, the galvanometer needle 36I so that'as the chopper bar 266 rises immediately subsequent to closure of switch 326, the galvanometer needle will function to actuate members 361 or 366 of the recording device. Condensers 336 (Fig. 8)- are connected through resistor 264, conductor 262, switch 254, and con ductors 256, 26I and 256 acrossthe galvanometer 256. In this instance these condensers are of the electrolytic type, and for this reason two condensers are employed in opposition. This condenser system acts as a storage reservoir for impulses received from the amplifier over conductors 235 and 236 and discharge through resistor 264 into the galvanometer coil 362 by way of the conductors above mentioned. This construction is employed because it is desirable to feed the galvanometer with a pulse whose character is determined by the time constant of the galvanometer, and this is accomplished by proportioning condensers336 and resistor 264 so as to materially prolong the excursion of the galvanometer needle 36I and allow maximum time for the chopper bar to close and grasp the deduring the opening of contacts 332 and the clos'-.

ing of contacts "I, cannot be reflected by any change in the position of galvanometer needle 36I sincethe needle is mechanically locked in position by the chopper bar during this portion of the cycle. Upon release of the galvanometer needle. contacts 33I have been closed and contacts 332 opened, and therefore, the galvanometer is thenconnected only to its critical dampening resistance and returns to the position of rest in the minimum time. Any electrical pulse generated in thef amplifier because of the opening of contacts 329are absorbed in resistance'331, and

by the time cain 321 is again advanced to a position to operate switch 326, the entire amplifier and galvanometer. system has returned to normal.

In operating the amplifier from ordinary voltage supplies, I have found it advisable to take certain precautions to stabilize the voltages fed to the amplifier circuit. In Fig. 9 I have shown a 110 volt 60 cycle power supply indicated by lines 313 connected through a switch 314 to primaries 315 and 318 of transformers 311 and 318. Condensers 319 and 38| are connected across the power supply lines 313 and carried to ground through conductor 382, these condensers being approximately 0.01 mfd. This compensates to some extent for high frequency transit potentials entering through the supply lines. Transformer 315 has a secondary 383 and may be any one of the so-called constant voltage type devices or any other device for delivering an output of substantially constant potential in spite of variable potential on the input. The filaments of all of the tubes in the cascade are fed from this transformer through conductors 384 and 385 returning to ground by way of conductor 339 and from ground returning to the secondary of the transformer by way of conductor 388. Transformer 318 has a secondary 381 arranged to feed the rectifier 345 through conductors 388 and 389, which serves as a source of power for relay 338. Another secondary 39| on the same transformer serves to energize the filament of a standard rectifier tube designated generally by the numeral 392, such as the tube known commercially as an "80 by way of conductors 393, 394 and 395. A third secondary 398 on the transformer 318 delivers high potential altemating current through conductors 391 and 398 to rectifier tube 392 where it is rectified, the positive terminal of said rectiiier delivering to filter choke 399, and the negative side of the rectifier system being connected by conductor 40l to ground by way of conductors 349, 348, 341 and 348. The point 482 on the output side of the filter 339 therefore becomes the so-called B plus of the rectifier system, and condenser 403 is connected between the conductor 394 and the ground wire 400, while a condenser 404 is connected between the point 402 and the ground wire 400, these connections being made by conductors 485 and 408.

The B plus voltage is delivered by way of conductor 401 to a point 408, and thence through resistors 409, 4 and H2 and conductor 413 to the plate 4 of the first tube of the cascade, the cathode of the tube 358 being connected to ground through resistance 5, conductors M8 and 4" to ground wire 352. This resistance should preferably be variable in order to simplify the initial adjustment of the amplifier. The screen grid 418 of the first tube is energized through conductor 9 and resistor 42I, and resistors 4H and 409 connected to the point 488 at which the B plus supply is connected. Connected at the point 422 between resistors 4H and 421 is a resistor 423, and connected to groimd through conductor 424 and variable resistance and utilized to develop on the resistance 4l8 a suflicient potential drop so that the cathode will be properly biased with respect to the grid 355 of the first tube. A resistor 425 connects between conductor 9 and the ground wires "8 and 411 acting as ableeder to stabilize the potential at the screen grid 8 to the required value. On the B plus supply between the resistors 489 and 4| l is connected avoltage regulator tube 428, the opposite side of this tube being returned to ground by connection to groimd conductor U8 and 4| 1 by a conductor 421. The tube 428 is of the gaseous dischargetypecharacterizedbysubstantially constant voltage drop across its terminals over wide ranges of current flow, its function being to stabilize the plate and screen potentials for the first tube. The function of resistor 4 is to produce the required voltage at the point 422.

A conductor 43! branches from conductor 3 and feeds through a condenser 432 and conductor 433 to grid 434 of the second tube 31L Potential increments appearing at conductor 3 consequently will be reflected in grid 434 of the second tube, this being a standard capacity connected amplifier circuit. Grid 434 of the second tube is also connected by conductor 435 and grid leak 438 to ground by conductors 431, 438 and 439. Resistor 44l constitutes a bleeder from positive B at point 402 through a conductor 442 to ground by way of a conductor 443, and is tapped at point 444, conductors 445 and 448 and resistance 441 delivering to the cathode 448 of said second tube, resistance 441 acting as a bleeder delivering to ground through a variable resistance 449 and conductors 458, 431, 438 and 439, the potential generated across resistance 449 constituting the grid bias on the second tube. A resistor 45| is connected at one end to conductor 449 and to the grid 452a of tube 311 by conductor 452, a bleeder 453 to ground insuring the proper potential on conductor 452. Conductor 448 is branched at the point 454 to form conductor 455, from which are fed resistors 458, 451 and 458. Resistor 458 constitutes the plate resistor of the second tube, being connected to the plate 459 thereof by conductor 48 l Variations in the potential of conductor 48l are consequently reflected by a condenser 482 connected thereto by conductor 480 and thence through a conductor 483 to grid 484 of the third tube 312, this also being a standard capacity coupling between tubes.

The ratio of resistance 441 to 449 is such that when a change in potential occurs between point 485 and ground, the increment of potential drop thereby reflected in resistance 449 will cause a change in bias on the second tube of such magnitude as to substantially compensate for the change in plate current in the second tube which would otherwise occur, that is, the resistances 441 and 449 are so proportioned that when change in potential at point 485 occurs, there will not occur a corresponding change in potential in conductor 452 so that changes in B voltage due to line voltage fluctuations will not be reflected by a change in potential of the grid of the third tube by virtue oi changes in potential occurring at point 488.

From a point 488 of bleeder resistor 4, conductor 481 delivers to resistors 488 and 489, and from resistor 488. conductor 4 branches at point 412 and delivers to resistor 413 and through a conductor 414 to condenser 334. From resistor 413 a conductor 415 delivers through resistor 458 to conductor 455. Grid 484 of the third tube is connected by a conductor 418 to grid leak 451 delivering to conductor 455. Potential drop through resistor 488 accordingly constitutes the bias on the grid of the third tube, and it will be apparent that the cathode 411 of the third tube connected thereto by conductor 418 operates at considerably above ground potential. The function of resistor 413 is dual in nature. It acts to supply current to resistor 458 via conductor 415 of such magnitude that the proper grid bias is applied to the third tube. Refer now to point 419 on conductor 481. This constitutes the point of connection of one conjugate conductor of a Wheatstone bridge, the point of connection of the other conjugate conductor being the point 48! at which conductor 415 meets conductor; 413.. The armsv of isaid..bridge then constitute first the resistor 463, second the primary 432 of a transformer 433 and .a condenser 434 in parallel and connected to the plate 435 by tube 312 by conductor 436, and said combination purpose of this bridge combination is to maintain points 412 and 431 at substantially the same potential with. changes of B supply at point 466, since in this manner condensers 333 and 334 are therefore both at substantially the same potential when no pulse is being amplified and there is accordingly substantially no tendency for leakage to occur through said condensers and into the galvanometer circuit.

While points 431 and 412 are normally at substantially the same potential, these potentials may not be exactly in balance, in which case serio'us error may occur if, concurrently, the ohmic leakage through condensers 333 and 334 allows a continuous current to flow through the galvanometer circuit when contacts 332 are closed. This possibility can be completely avoided in the manner shown in Fig. 11 wherein dual condensers 333 and 333a replace condenser 333 on conductor 336 while 334 and'334a replace condenser 334 on conductor 335. A high resistance 343 connected between the junction of 333 and 333a and the junction of 334 and 334a effectively bleeds any ohmic leakage away from the galvanometer circuit without appreciably interfering with the effectiveness of the condenser system in transmitting pulses. This bridge circuit may be explained in. another way by saying that resistances 463, 413, 463, etc., are so proportioned that the potential at 412 is normally the same as the potential at 431, under which circumstances the plates of the condensers connected to these points are at substantially the same potential, and consequently there occurs no tendency for leakage through the galvanometer when connected to the opposite plates of condensers 333 and 334. Conductor 3 leading to grid 3 of the first tube is connected to a condenser 433, the opposite side of said condenser being connected to the filament circuit of the same tube, and since this filament circuit is supplied with alternating current, there is impressed on grid 3 an alternating potential via condenser 433. This potential is amplified in the first, second and third tubes, and appears as amplified A. C. on the plate of the third tube where it is fed to the combination of primary inductance 432 and associated condenser 434 of a transformer 433.

This inductance and capacitance are so proportioned that their parallel combination exerts a maximum impedance to the particular frequency fed through the condenser 433, and it will be recognized that this particular frequency is far removed from the frequency of the pulse generated in the cascade when relay member '362 is moved from contact 363 to contact 361. Consequently, while the combination acts as a very high impedance to the signal entering the circuit through condenser 433, it does not materially impede the pulse signal generated by closure of the relay. The secondary 433 of the transformer: consequently. continuously deliver alternating currentaof the frequency pasled .by condenser 433 when the amplifier is functioning normally. One side of-the secondary 433 is connected to ground by conductors I and 333 and'a conductor 432 connects to the opposite side of said secondary and delivers to grid 4" of a fourth vacuumtube 434. The plate 433 of this tube is connected'to B voltage at 433 by a conductor 436 and'a plate resistor 431, while a bleeder 433 connected between the point 433 and ground by conductor 433 is tapped at the point 531 and connected by conductor 532, to screen grid 533 in the same tube. Bleeder 433 is further tapped at point 534 and is connected from this point to the cathode 535 of the tube by conductor 536, thereby supplying the proper bias with re.- spect to grid. A condenser 531 connected between the cathode and ground wire 433 further improves the gain of this tube by reducing cath- 436 is a condenser 533 connected in series with a glow lamp such as a neon bulb 533 and thence to ground by conductors 5H and 433. It will be apparent, therefore, that when the amplifier system is functioning normally, the alternating ,current impressed through condenser 433 onto screen grid 3 of the first tube will be amplified successively in the first, second, third and fourth tube and will appear as a greatly amplified A. C. signal of the same frequency on the plate lead 436 of the fourth tube. From this point the alternating current component will pass con.- denser 533 and cause the neon light 533 toglow. If, however, any tube in the circuit becomes inoperative or if any other element of the amplification system fails, the alternating signal impressed by condenser 433 will fail to amplify and appear at neon bulb 533, and failure of the amplifier will thereby be immediately evidenced. Further, bulb 533 being a neon lamp requires a minimum voltage before it begins to glow so that in the event gradual failure of overall amplification factor of the cascade occurs, the neon light will continue to light until the amplification factor of the cascade falls to a predetermined minimum, at which point the light will sharply go out. The signal circuit, therefore, gives a sharp indication when the overall amplification factor of the cascade falls below a preselected minimum. This is a very important factor in industrial equipment, since there is immediate response in the event of failure of any element in the circuit. Further, in the event large potentials are impressed on grid 355 by virtue of ex treme out-of-balance conditions between the. potentiometer and the cell chain and its associated temperature compensators, there will occur momentary blocking of thegrid in one or more of the first three tubes' in the cascade, and;

such blocking will immediately be evidenced by failure of the signal light. The light, therefore, discloses not only satisfactory operation of the amplifier itself, but also indicates if abnormal conditions 'exist in the potentiometer or cell chain circuit, such, for example, as afailure of the glass electrode, failure of a control valve, shorted leads, etc.

Referring further to the input circuit of the first tube, resistance 333 is of high impedance and by-passing this resistance to the McClure circuit is a condenser 5l2. This combination constitutes a filter circuit such as that described in detail in connection with Fig. 1, resistance ,3" of Fig.3 oorrespondingto resistance 223 of l'ig.

1. and condenser III of Fig. 9 corresponding to condenser 24! of Fig. 1. This filter circuit serves to prevent the entrance of the amplifier of extraneous stray signals generated in conductor I13 or in parts of the potentiometer call chain circuit.

It will be recognized by those skilled in the art that the amplifier system herewith disclosed need not be restricted to the particular number of tubes illustrated. In certain instances a lesser number of tubes may be satisfactory, while in other cases more tubes might be employed and such other combinations may be made within the scope of this invention.

Referring now more particularly to Fig. 12, I have therein shown power operated means for cleaning the electrode, to which device reference has already been made. In this construction the brush II is caused to extend from a point above the conductive layer 63 to a point below the conductive layer 68 and the handle or stem thereof extends upward through the fitting 34, a bushing III being positioned in the bore 31 for the reception of a bearing M6 on the brush stem. A motor ill of conventional design is supported on the fitting 34 by means of a bracket H8 and screws M3, the bracket and screws being insulated from the bushing by means of insulated plates 52L The bearing member SIG is connected to and driven by the motor. Interposed between the motor and a source of power 522 are switch contacts 523 positioned on switch 328 (Fig. 8) and actuated by means of the cam 32'! in such manner that the contacts 523 are closed during the same period as the contacts 33I and opened when these contacts are open so that the motor operates to rotate the brush during the interval between tests. The density of the bristles on the brush 1| must, of course, be such as not to materially interfere with the fiow of sample through the electrode.

In Fig. 13 I have shown means for automatically replacing the liquid junction at 63 (Figs. 3 and 4). This device includes a cylinder 524 having a threaded outlet end 525 somewhat similar in nature to the threaded outlet 89 of tube 9i (Fig. 3) adapted to be threaded into the member 81 in place of the collapsible metal tube 9| of Fig. 3 and adapted to hold a supply of bridge material indicated at 526. A plunger 521 is positioned in the cylinder 524 and has a threaded stem 528 upon which is positioned a worm wheel 529 secured against lateral movement and rotated by means of a reduction gear train indicated generally by the numeral 53! and driven by a motor 532 connected to a source of power by leads 533. If desired, switch contacts similar to contacts 523 of Fig. 12 may be interposed in this power supply for the purpose of intermittently actuating the motor and feeding the bridge material to the liquid junction in a step-by-step fashion. As shown, the mechanism continuously feeds bridge material, the gear train 53l being such as to very slowly advance the plunger 52'! and continuously replace the bridge material.

While any suitable form of bridge material may be employed, I have found it particularly desirable to employ a novel type of bridge material in the form of a plastic composition incapable of flowing under its own hydrostatic pressure. A suitable material is formed by heating together one hundred grams of water, two 1 grams of agar over a steam bath until the mixture is clear. Thereupon about thirty grams of potassium chloride is dissolved therein and the mass allowed to cool and remain quiescent until a gel is well established. The gel mass is then forced through a mesh screen to rupture the gel and render the mass plastic. This latter step breaks up the gel into small particles rendering the same sufllclently plastic to flow under moderate pressure. I have found, however, that a preferred type of composition results from the use in the combination of a plasticizer such as glycerol, ethylene glycol, diethylene glycol, or sulphonated castor oil. This is desirable since it avoids the difllculty heretofore associated with the use of bridge materials in their tendency to seep out and dry around the edges of a stop cock or at the junction of tubes. When such a plastlcizer is used the gel remains permanently plastic and the above mentioned diiliculties are not encountered. This preferred type of composition can be made as follows: About 30 grams of agar and 180 cc. of water are heated over a water bath until clear. About 2 grams of methyl cellulose is then added to the solution near its boiling point and allowed to cool with stirring until the cellulose peptizes and the solution becomes smooth and free of lumps. About 20 cc.

of glycerine are then added and the mass reheated, whereupon about 60 grams of potassium chloride and about grams of barium sulphate are added and the mass stirred until the chloride is dissolved and the sulphate is well dispersed. Material is then cooled and well stirred just before the setting temperature is reached. The material is then allowed to stand to produce a gel and when gelled, is forced through a mesh screen having a mesh size in accordance with the degree of plasticity required, approximately thirty mesh screen being satisfactory for use in the apparatus herein disclosed.

While I have thus described and illustrated specific embodiments of the invention it will be understood that this is by way of illustration and not limitation, and I do not wish to be limited except as required by the prior art and the scope of the appended claims, in which-- I claim:

1. The combination in an ion activity measurement device of an electrolytic cell chain for producing a potential proportional to an electrochemical characteristic of a test sample, means for applying a correction potential to the potential of said cell chain to correct the same for variations thereof due to changes of said characteristic caused solely by variation in the temperature of saidsample from a reference temperature, and means for measuring the modified output of said cell chain.

" 2. The combination in an ion activity measurement device of an electrolytic cell chain for producing a potential proportional to a characteristic of a test sample, means for applying a correction potential to the potential of said cell chain to correct the same for variations of said characteristic due to variation in the temperature of said sample, means for measuring the modified output of said cell chain, and means for modifying said last mentioned means to produce a response proportional to the absolute temperature of said sample to measure said characteristic of said sample.

3. The combination in an ion activity measurement device of an electrolytic cell chain for producing a potential proportional to an electrochemical characteristic of a test sample at a pre-selected temperature, means for applying a correction potential to the potential of said cell due to changes of said characteristic caused solely by variation in temperature of said sample from said temperature, and means including an electronic amplifier for measuring the modified output of said cell chain to measure said characteristic.

4. The combination in an ion activity measurement device of an electrolytic cell chain for producing a potential proportional to a characteristic of a test sample, means for applying a fier for determining the point of balance of said last mentioned means, to measure said unknown potential includinggcontact means for completing electrical'circuit between said source and said balancing means exclusive of said amplifier for producing a measurement impulse, a condenser shunting said contact means for reducing extraneous contact potentials, and means comprising a filter network between said source and said contact means for preventing stray field interference from entering the amplifier.

6. The combination in an ion activity measurement device, or an electrolytic cell chain for producing a potential proportional to a characteristic of a test sample including a test electrode and a reference electrode, means for applying a correction potential to the potential of said cell chain to correct the same for variations of said characteristic due to change in temperature or said sample, means for applying a correction potential to the potential of said cell chain for variations therein due to change in temperature of said reference electrode, and means for, measuring the corrected output of said cell chain to measure said characteristic.

'1. The combination recited in claim 1 including means for indicating the degree of correction potential applied to the'potential of said cell chain.

8. The combination in an ion activity measurement device, of an electrolytic cell chain for producing a potential proportional to a characteristic of a test sample including a test electrode and a reference electrode, means for applying a correction potential to the potential of said cell chain to correct the same for variations of said characteristic due to change in temperature of said sample, means including an electronic amplifier for measuring said corrected potential, a conductor between said test electrode and said last mentioned means, a shield for said conductor, and means to bringthe shield to substantially the potential of said conductor to prevent leakage between said conductor and shield during measurement.

9. The combination in an ion activity measurement device of an electrolyte cell chain for producing a potential proportional to a characteristic oi! a test sample including a test electrode, means including an electronic amplifier for measuring said potential, a conductor bet ampere; chain to correct the same for variations-thereof tween said test electrode and said-last 'men-" tioncd means, a shield for said conductona cir cuit including said shield for bringing the latter to the same potential as said conductor during measurement, and means in said conductor substantially at said amplifier for preventing stray potentials on said conductor from entering said amplifier comprising a high impedance in saidconductor, and a condenser connected to said conductor between said impedance and said test electrode and connected to said circuit.

10. The combination in an ion activity measurement device, of an electrolytic cell chain for producing a potential proportional to a characteristic of a test sample including a test electrode, means for applying a correction potential to'the potential of said cell chain to correct the same for variations of said characteristic due to change in temperature of said sample, means including an electronic amplifier for measuring said corrected potential, a conductor between said test electrode and said last mentioned means, a shield for said conductor, a circuit ineluding said shield for bringing the latter to the same potential as said conductor during measurement, and means in said conductor substantially at said amplifier for preventing stray potentials on said conductor from entering said amplifier comprising a high impedance in said conductor and a condenser connected to said conductor between said impedance and said test electrode and connected to said circuit.

11. The combination in a hydrogen ion measurement device of a cell chain including a glass electrode comprising a glass tube, a temperature tube located adjacent thereto, means for circulating a test sample through said glass electrode and said temperature tube, means iorapplying a correction potential to the potential of said cell chain to correct the same for variations in pH due to change in temperature of said sample, said means including a temperature responsive element in thermally responsive relationship with said temperature tube, means for bal-v ancing a known potential against the modified output of said cell chain, means for further applying to said balancing means a potential proportional to the absolute temperature of said glass electrode comprising a temperature responsive element in thermally responsive relationship with said temperature tube, and means for determining the point of balance of said balancing means to measure the pH of said sample.

12. The combination in an ion activity measurement device of an electrolytic cell chain for producing a potential proportional to a, characteristic of a test sample, means for balancing a known potential against theoutput of said cell chain, an impulse amplifier ohmically. isolated and electrostatically connected to said cell chain for determining the point of balance of said last mentioned means, and means for periodically applying to said amplifier any unbalanced potential from said balancing means, adjusting said balancing means to the point of balance,

a and recording said point of balance to indicate and record changes in said characteristic.

13. The combination in an ion activity measurement device of an electrolytic cell chain, means for balancing a known potential against the output of saidvcell chain, an impulse amplifier ohmically isolated and electrostatically connected to said cell chain responsive to the condition of balance oi.said balancing means, and

means for applying to the amplifier any unbal- 

